1. Field of the Invention
The present invention relates to an optical waveguide device used, e.g., in the fields of optical information processing and optical application measurement and control, and to a light source and an optical apparatus using the same.
2. Description of the Related Art
Optical information recording/reproducing apparatuses can achieve higher density by using a shorter-wavelength light source. For example, a widespread compact disk (CD) apparatus uses near-infrared light having a wavelength of 780 nm, while a digital versatile disk (DVD) apparatus that can reproduce information with higher density uses a red semiconductor laser having a wavelength of 650 nm. To achieve a next-generation optical disk apparatus with even higher density, a blue laser source with even shorter wavelength has been under active development. For example, to provide a small and stable blue laser source, a second harmonic generation (hereinafter, referred to as xe2x80x9cSHGxe2x80x9d) device has been developed by using a nonlinear optical material. For convenience, harmonic light that is produced by the SHG device also referred to as xe2x80x9cSHG lightxe2x80x9d in the following.
FIG. 22 is a schematic view showing an example of an SHG blue light source including an SHG device.
First, the SHG blue light source will be described by referring to FIG. 22.
As shown in FIG. 22, a high refractive index region with a width of about 3 xcexcm and a depth of about 2 xcexcm is formed on an optical material substrate 114 by a proton-exchange method. This high refractive index region functions as an optical waveguide 110. Infrared light with a wavelength of 850 nm emitted from a semiconductor laser 111 is focused on the entrance end face of an SHG device 117 and propagates in the optical waveguide 110 on the SHG device 117 so as to be a fundamental guided wave. LiNbO3 crystals, which are used as a substrate material for the optical material substrate 114, have a large nonlinear optical constant. Therefore, a harmonic guided wave having half the wavelength of the fundamental light (i.e., 425 nm) is excited from the electric field of the fundamental light. To compensate for a difference in propagation constant between the fundamental light and the harmonic light, a periodic polarization inversion region 112 is formed on the optical waveguide 110. The harmonic light that is excited over the entire region of the optical waveguide 110 is added coherently, which then exits from the exit end face of the optical waveguide 110.
It is necessary to maintain the wavelength of the fundamental light precisely constant to ensure accurate compensation for the difference in propagation constant between the fundamental light and the harmonic light. Therefore, a distributed Bragg reflection (hereinafter, referred to as xe2x80x9cDBRxe2x80x9d) semiconductor laser is used as the semiconductor laser 111. The DBR semiconductor laser includes a DBR region and shows extremely small wavelength variations with respect to temperature or the like. In addition to such small wavelength variations, the DBR semiconductor laser also is characterized by high coherence and small noise because it oscillates with a single wavelength.
Next, the operation of an optical pickup system that includes the SHG blue light source using the SHG device will be described by referring to FIG. 22.
As shown in FIG. 22, harmonic blue light emitted from the SHG device 117 passes through a collimator lens 119, a polarizing beam splitter 120, a quarter-wave plate 121, and an objective lens 122 in sequence, and then is focused on an optical disk 124. The light modulated by the optical disk 124 is reflected from the polarizing beam splitter 120 and directed to a photodetector 125 through a focusing lens 123, thus providing a reproduction signal. At this time, linearly polarized light emitted from the SHG device 117 in the direction parallel to the sheet of the drawing is polarized in the direction perpendicular thereto by passing through and returning to the quarter-wave plate 121. All the reflected light from the optical disk 124 is reflected by the polarizing beam splitter 120 and does not return to the light source side.
In the aforementioned conventional technique, a configuration in which all the reflected light from the optical disk 124 is reflected by the polarizing beam splitter 120 and does not return to the light source side has been described. However, the base material for actual optical disks has a birefringent property. Thus, undesired polarized components generated in the optical disk 124 may pass through the polarizing beam splitter 120 and return to the light source side, as indicated by return light 126. During reproduction of the optical disk 124, the position of the objective lens 122 is controlled so as to ensure precise focusing on the optical disk 124. Therefore, the exit end face of the SHG device 117 and the optical disk 124 constitute a confocal optical system, and the reflected light from the optical disk 124 is focused precisely on the exit end face of the SHG device 117 (i.e., the exit end face of the optical waveguide 110).
As described above, the reflected light that returns from the optical disk to the light source side becomes return light to induce noise in the optical system using a semiconductor laser as the light source, and various techniques for avoiding this have been proposed. Examples of such techniques include a method for generating a plurality of longitudinal modes by modulating a semiconductor laser with a high frequency signal and a method for also generating a plurality of longitudinal modes by causing self-oscillation in a semiconductor laser. In the field of optical communication, an optical isolator that has a magneto-optical effect generally is located between a semiconductor laser and an optical fiber so that light from the semiconductor laser is focused on the optical fiber. Moreover, another method has been proposed that prevents reflected light from returning to a semiconductor laser by cutting the entrance end face of an optical fiber or an optical waveguide so as to reflect the reflected light that returns from the optical disk to the light source side obliquely (JP 5(1993)-323404 A or the like).
These techniques reduce noise caused by return light that returns to the inside of a semiconductor laser. As a result of experiments on reproduction of the optical pickup that includes the optical waveguide type SHG device shown in FIG. 22, the present inventors found noise caused by a different mechanism from that of the conventional noise induced by the return light. This noise is interference noise to be generated when the return light focused on the exit end face of the optical waveguide is reflected therefrom and interferes with light emitted from the optical waveguide. The output power of the light source appears to change due to this interference effect from the optical disk side, and a reproduction signal of the optical disk is modulated by low frequency noise, which leads to degradation of the reproduction signal. The noise induced by the return light in the semiconductor laser is generated by the interaction between light inside the semiconductor laser and the return light. On the other hand, the interference noise is generated by the interference between light emitted from the light source and the return light. More detailed studies conducted by the present inventors showed that a portion of the return light from an external optical system is excited again in the optical waveguide of the SHG device (the optical waveguide device) as a guided wave, then is reflected from the entrance end face of the optical waveguide, and also causes interference noise.
As described above, there are two different types of noise in the optical system that uses the optical waveguide device: low frequency interference noise and mode hopping noise. The low frequency interference noise occurs when light emitted from a light source is reflected and returns to the exit end of the light source to cause interference in the optical system outside the light source. The mode hopping noise results from the inside of the semiconductor laser. Various techniques have been proposed as a method for reducing the mode hopping noise. However, so far no consideration has been given to the interference noise outside a light source, and no solution to this problem has been proposed.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an optical waveguide device that can reduce external interference noise, a light source and an optical apparatus using the optical waveguide device.
To achieve the above object, an optical waveguide device according to a configuration of the present invention includes a substrate, an optical waveguide formed in the vicinity of the surface of the substrate, and an optical thin film formed in at least a portion of the optical waveguide or formed in contact with at least a portion of the optical waveguide. A plurality of guided waves with different wavelengths propagate in the optical waveguide, a portion of the guided waves that exits from the optical waveguide returns to the optical waveguide again, and the optical thin film prevents reflection of light returning to the optical waveguide from the end faces of the optical waveguide or in the vicinity thereof.
In the configuration of the optical waveguide device of the present invention, it is preferable that fundamental light and harmonic light propagate in the optical waveguide, a harmonic light absorption region that includes the optical thin film and absorbs the harmonic light is provided in the vicinity of the entrance end face of the optical waveguide, and the fundamental light propagates in the harmonic light absorption region and a harmonic light non-absorption region that does not absorb the harmonic light in the optical waveguide, and waveguide mode sizes of the fundamental light in the two regions substantially match. According to this preferred example, when a light source is formed by combining with a semiconductor laser and this light source is used, e.g., in an optical pickup, interference noise caused by multiple reflection between an object to be observed, such as an optical disk, and a reflecting plane in the light source can be reduced, and wavelength conversion from the fundamental light into the harmonic light can be performed efficiently.
It is preferable that the effective refractive index of the harmonic light absorption region substantially matches that of the harmonic light non-absorption region. According to this preferred example, the waveguide mode sizes of the fundamental light propagating in the harmonic light absorption region and the harmonic light non-absorption region can be matched substantially.
It is preferable that the fundamental light propagates in both the harmonic light absorption region and the harmonic light non-absorption region with a zero-order transverse mode.
It is preferable that a harmonic light absorption film that acts as the optical thin film is formed on the optical waveguide in the harmonic light absorption region.
It is preferable that the optical waveguide in the harmonic light non-absorption region includes an optical waveguide layer and a high refractive index layer formed on the optical waveguide layer, in which the refractive index of the high refractive index layer is larger than that of the optical waveguide layer, and the optical waveguide in the harmonic light absorption region includes a second optical waveguide layer and a harmonic light absorption layer formed on the second optical waveguide layer, in which the harmonic light absorption layer includes a harmonic light absorption film as the optical thin film, and the refractive index of the harmonic light absorption film is larger than that of the second optical waveguide layer.
It is preferable that the optical waveguide in the harmonic light non-absorption region includes an optical waveguide layer, and the optical waveguide in the harmonic light absorption region includes a second optical waveguide layer and a harmonic light absorption layer formed on the second optical waveguide layer, in which the upper portion of the second optical waveguide layer is removed so as to have a smaller thickness than that of the optical waveguide layer, the harmonic light absorption layer includes a harmonic light absorption film as the optical thin film, and a refractive index of the harmonic light absorption film is larger than that of the second optical waveguide layer.
It is preferable that the optical waveguide in the harmonic light non-absorption region includes an optical waveguide layer, and the optical waveguide in the harmonic light absorption region includes a second optical waveguide layer and a harmonic light absorption layer formed on the second optical waveguide layer, in which the substantial thickness of the second optical waveguide layer is smaller than that of the optical waveguide layer, the harmonic light absorption layer includes a harmonic light absorption film as the optical thin film, and the refractive index of the harmonic light absorption film is larger than that of the second optical waveguide layer.
It is preferable that the optical thin film is formed by mixing and diffusing metal in the optical waveguide in the harmonic light absorption region. This preferred example can eliminate unevenness on the substrate surface caused by forming the harmonic light absorption film. Therefore, the configuration is suitable for precise face down mounting with the waveguide plane oriented toward the mounting substrate. In this case, iron can be used as the metal.
It is preferable that the fundamental light propagates in the harmonic light non-absorption region with a zero-order transverse mode and in the harmonic light absorption region with a higher-order transverse mode of not less than first order.
It is preferable that the optical waveguide in the harmonic light absorption region includes an optical waveguide layer and a harmonic light absorption layer formed on the optical waveguide layer, in which the harmonic light absorption layer includes a harmonic light absorption film as the optical thin film, and the refractive index of the harmonic light absorption film is larger than that of the optical waveguide layer.
It is preferable that the optical waveguide in the harmonic light absorption region includes an optical waveguide layer, a high refractive index layer formed on the optical waveguide layer, and a harmonic light absorption layer formed on the high refractive index layer, in which the refractive index of the high refractive index layer is larger than that of the optical waveguide layer, the harmonic light absorption layer includes a harmonic light absorption film as the optical thin film, and the refractive index of the harmonic light absorption film is larger than that of the high refractive index layer, and the optical waveguide in the harmonic light non-absorption region includes a second optical waveguide layer and a second high refractive index layer, in which the refractive index of the second high refractive index layer is larger than that of the second optical waveguide layer.
It is preferable that the optical waveguide in the harmonic light absorption region includes an optical waveguide layer, a harmonic light absorption layer formed on the optical waveguide layer, and a high refractive index layer formed on the harmonic light absorption layer, in which the harmonic light absorption layer includes a harmonic light absorption film as the optical thin film, the refractive index of the harmonic light absorption film is larger than that of the optical waveguide layer, and the refractive index of the high refractive index layer is larger than that of the optical waveguide layer, and the optical waveguide in the harmonic light non-absorption region includes a second optical waveguide layer and a second high refractive index layer, in which the refractive index of the second high refractive index layer is larger than that of the second optical waveguide layer.
It is preferable that the optical thin film is formed by mixing and diffusing metal in the optical waveguide in the harmonic light absorption region.
It is preferable that the waveguide mode size of the fundamental light propagating in the optical waveguide in the harmonic light absorption region changes continuously, and the waveguide mode sizes of the fundamental light at the entrance end and the exit end of the harmonic light absorption region substantially match the waveguide mode size of the fundamental light propagating in the optical waveguide in the harmonic light non-absorption region. According to this preferred example, the fundamental light can propagate efficiently, thus providing the harmonic light efficiently.
It is preferable that the optical waveguide in the harmonic light absorption region includes an optical waveguide layer and a harmonic light absorption layer formed on the optical waveguide layer, in which the harmonic light absorption layer includes a harmonic light absorption film as the optical thin film, and the substantial thickness of the harmonic light absorption film changes continuously in the harmonic light absorption region.
In the configuration of the optical waveguide device of the present invention, it is preferable that fundamental light and harmonic light propagate in the optical waveguide, and a harmonic light absorption film that absorbs the harmonic light and acts as the optical thin film is formed at the entrance end face of the optical waveguide. According to this preferred example, when a light source is formed by combining with a semiconductor laser and this light source is used, e.g., in an optical pickup, interference noise caused by multiple reflection between an object to be observed, such as an optical disk, and a reflecting plane in the light source can be reduced, and wavelength conversion from the fundamental light into the harmonic light can be performed with high efficiency.
It is preferable that the harmonic light absorption film is a semiconductor film or an oxide dielectric film. In this case, it is preferable that the semiconductor film is one selected from the group consisting of a Si film, a ZnSe film and GaP film, and the oxide dielectric film is a TiOx film. It is preferable that the TiOx film satisfies 1.7 less than x less than 1.9.
In the configuration of the optical waveguide device of the present invention, it is preferable that the magnitude relationship between N1 and N2, i.e., which of N1 and N2 is greater, differs depending on a wavelength of the guided wave, where N1 is a refractive index of the optical thin film and N2 is a refractive index of the optical waveguide. This preferred example can separate light propagating in the optical waveguide depending on the wavelength. In particular, the influence of return light can be reduced effectively by a simple structure in which the optical thin film is formed in a portion of the optical waveguide. By using the optical thin film whose magnitude relationship in refractive index with the optical waveguide is reversed depending on wavelength, it is possible, in principal, to reduce the influence of the optical thin film upon the guided wave having one wavelength and to increase the influence of the optical thin film upon the guided wave having the other wavelength. In this manner, the guided waves propagating in the optical waveguide can be separated efficiently by wavelength.
It is preferable that the refractive indexes for at least one of the guided waves satisfy N1 greater than N2, and the refractive indexes for the other guided wave satisfy N1 less than N2. In this case, it is preferable that fundamental light having a wavelength of xcex1 and harmonic light having a wavelength of xcex2 propagate in the optical waveguide, and the wavelengths satisfy xcex greater than xcex2, the refractive indexes for the fundamental light having the wavelength xcex1 satisfy N1 less than N2, and the refractive indexes for the harmonic light having the wavelength xcex2 satisfy N1 greater than N2. According to this preferred example, light having a short wavelength can be trapped within the optical thin film, while the light having a long wavelength can be propagated in the optical waveguide. Moreover, it is preferable that the optical thin film has an absorption coefficient of not less than 0.001 for the harmonic light. According to this preferred example, SHG light can be attenuated to {fraction (1/10)} or less by the optical thin film of about several hundred micrometers length. It is preferable that a harmonic light absorption film for absorbing the harmonic light is further provided on at least one selected from the upper face, the lower face, and the inside of the optical thin film.
It is preferable that the optical thin film is formed of a multi-layer film. The refractive index dispersion of the optical thin film is inherent in its material. Therefore, it is difficult to adjust it precisely. However, this preferred example can control the dispersion characteristics of the film easily, so that the dispersion relationship between the optical waveguide and the optical thin film can be adjusted.
It is preferable that the optical thin film is formed on at least one selected from the surface, the lower face, and the side faces of the optical waveguide.
It is preferable that the optical thin film is formed in the vicinity of the entrance portion of the optical waveguide. This preferred example can improve the efficiency of the SHG device. That is, SHG light increases in proportion to the square of the distance between the entrance portion and the exit portion. By forming the optical thin film in the vicinity of the entrance portion, the influence upon the SHG light that increases with distance can be suppressed, thereby achieving high efficiency.
It is preferable that normal lines to the end portions of the optical thin film tilt with respect to the propagation direction of the guided wave at an angle greater than zero. This preferred example can reduce reflection from the end portion of the optical thin film significantly, thus preventing noise caused by return light.
It is preferable that a clad layer having a refractive index N3 is further provided on the surface of the optical waveguide, and the refractive indexes of the clad layer and the optical waveguide satisfy N3 greater than N2. This preferred example can reinforce the confinement of the optical waveguide. Therefore, by applying this to the SHG device, the confinement of the optical waveguide can be reinforced to achieve high conversion efficiency.
It is preferable that an antireflection film is provided on the entrance en harmonic light. In this case, it is preferable that fundamental light and harmonic d face. This preferred example can further reduce a reflectance of the light propagate in the optical waveguide, and the antireflection film reduces a reflectance of the harmonic light to 1% or less.
A light source according to a first configuration of the present invention includes a semiconductor laser and an optical waveguide device. An optical waveguide device according to claim 1 is used as the optical waveguide device.
In the first configuration of the light source of the present invention, it is preferable that the semiconductor laser is a wavelength-variable semiconductor laser that has the function of varying wavelength with high coherence.
A light source according to a second configuration of the present invention includes a semiconductor laser having a wavelength of xcex1 and an optical waveguide device for converting light emitted from the semiconductor laser into light having a wavelength of xcexs. An antireflection film for light having the wavelength of xcexs is provided at the exit end face of the semiconductor laser. This second configuration of the light source can reduce the influence of return light effectively. In principle, when light with the wavelength of xcexs emitted from the optical waveguide device is returned by reflection of some kind in the optical system, the antireflection film can prevent reflection of this xcexs wavelength light, i.e., return light. Thus, it is possible to prevent the return light from being reflected again and causing interference noise in the optical system.
In the second configuration of the light source of the present invention, it is preferable that an antireflection film for light having the wavelength of xcexs or an antireflection film for light having the wavelength of xcex1 and light having the wavelength of xcexs is provided on at least one selected from the entrance end face and the exit end face of the optical waveguide device.
In the second configuration of the light source of the present invention, it is preferable that the exit end face of the optical waveguide device tilts with respect to the propagation direction of a guided wave.
In the second configuration of the light source of the present invention, it is preferable that the vicinity of the exit end face of the semiconductor laser is made of a material that absorbs light having the wavelength of xcexs.
In the second configuration of the light source of the present invention, it is preferable that the semiconductor laser has a grating structure.
In the second configuration of the light source of the present invention, it is preferable that the semiconductor laser and the optical waveguide device are coupled directly.
In the second configuration of the light source of the present invention, it is preferable that the antireflection film reduces a reflectance of light having the wavelength xcexs to 1% or less.
A light source according to a third configuration of the present invention includes a plurality of semiconductor lasers with different wavelengths and an optical waveguide device for converting light having wavelengths of xcex1, xcex2, xcex3, . . . , xcexn emitted from the respective semiconductor lasers into light having wavelengths of xcexs1, xcexs2, xcexs3, . . . , xcexsn. An antireflection film for the light having at least one wavelength selected from xcexs1, xcexs2, xcexs3, . . . , and xcexsn is provided on the exit end face of at least one of the semiconductor lasers.
An optical apparatus according to a configuration of the present invention includes a light source and a focusing optical system for focusing light emitted from the light source on an object to be observed. A light source according to the present invention is used as the light source, and the optical waveguide device of the light source and the object to be observed are arranged so as to have a confocal relationship. According to this configuration, light can be focused on the object to be observed, such as an optical disk, by the optical system having a simple configuration. Moreover, since a light spot can be maintained stably in the range of depth of focus by the confocal optical system, a stable optical system can be provided. Further, focus detection or the like can be applied, thus stabilizing the optical system. It is also possible to reduce interference noise caused by multiple reflection between the object to be observed such as an optical disk and a reflecting plane in the light source. This configuration can prevent return light from an external optical system from being reflected by the entrance end face of the optical waveguide and also can reduce the generation of return light that is directed to the semiconductor laser.
In the configuration of the optical apparatus of the present invention, it is preferable that the object to be observed is an optical disk.
In the configuration of the optical apparatus of the present invention, it is preferable that the apparatus further includes an optical fiber, and light emitted from the light source enters the optical fiber.